Labour MP Lucy Powell has resigned as shadow education secretary, saying that Jeremy Corbyn's position as leader of the party is "untenable".
In a letter to the Labour Party leader, she said: "It is increasingly clear that your position is untenable and that you are unable to command the support of the Shadow Cabinet, the Parliamentary Labour Party and, most importantly, the country.
Given the big challenges faced by our country and our Party over the coming months, and the possibility of an imminent general election, I do not have the confidence that you can bring the Party together, lead us to a General Election and be an effective Opposition.
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Shadow Scottish secretary Ian Murray announced his resignation from the shadow cabinet.
In a letter addressed to Jeremy Corbyn, he said the leader of the Scottish Labour Party, Kezia Dugdale is doing "an excellent job but needs a strong UK Labour Party to support her".
I believe that we can only win again as a party when the UK Labour Party is able to articulate the case for progressive policies the length and breadth of the United Kingdom. I also believe we can only do that under new leadership.
Mr Murray also said he felt the way some of his colleagues have been treated "is unfortunate".
"I will therefore return to the backbenches to serve my constituents and fight Scotland's corner from there," he added.
Mr Murray joins Heidi Alexander and Gloria De Piero in stepping down in the wake of the EU referendum, while Hilary Benn was sacked.
As the only Labour MP north of the border, means the replacement shadow Scottish secretary will have to be a non-Scottish MP.
Union members on Labour's National Executive are set to call for unity and confirm their support Jeremy Corbyn's leadership, sources said.
A statement is expected later amid union anger at the moves by members of the shadow cabinet against the Labour leader.
Jennie Formby, one of Unite's representatives on the National Executive Committee (NEC) said it was "nonsense " to blame Mr Corbyn for the referendum result and said Labour MPs should show unity.
Gloria De Piero has reportedly resigned from her position in Labour's shadow cabinet.
The shadow minister for young people and voter registration joins former health secretary Heidi Alexander in stepping down.
Shadow foreign secretary Hilary Benn was sacked by leader Jeremy Corbyn overnight.
So far Hilary Benn, Heidi Alexander and Gloria Del Piero have all resigned from the Shadow Cabinet.
Heidi Alexander has spoken about her difficult decision to resign as shadow health secretary.
She told ITV's Peston on Sunday "the only decent and honourable thing to do was to resign" as she no longer had faith in Jeremy Corbyn's leadership.
The MP said she asked herself could she "hand on heart" say that she felt that "Jeremy was the best person to be leading the Labour Party in developing the answers the country is demanding".
"And I didn't feel I could so that," Ms Alexander added.
When asked if she had any sense of any other potential cabinet resignations amid mass speculation that up to half of Corbyn's top team could be about to quit, Ms Alexander said "there are a few people who feel similarly to me".
Ed Balls said Labour needs a new leader as he accused Jeremy Corbyn of alienating "both sides" in the EU debate.
He told Peston on Sunday that the party "needs a change" to show it has a vision for the future in a general election that could take place within months.
"At a time when you have a big divide between urban Britain which wanted in and heartlands an rural Britain that wanted out, Jeremy Corbyn has actually managed to alienate both sides," he said.
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John McDonnell insisted he will "never" stand for the Labour leadership as he said the party should stand by current head Jeremy Corbyn.
Labour MPs were being canvassed over their support for the shadow chancellor in a potential leadership takeover, ITV News understands.
However Mr McDonnell said he had no desire to take over the top job in an interview with the BBC's Sunday Politics programme, adding: "Jeremy's not going anywhere".
I will never stand for the leadership of the Labour Party. If Jeremy has to stand for another leadership election, I will chair his campaign and I think the Labour Party members will elect him again.
Labour MPs are being asked to support shadow chancellor John McDonnell in a potential leadership takeover, ITV News understands.
A source within the party said shadow chief secretary to the Treasury, Seema Malhotra, has been ringing around to try to canvas support.
Reports suggest she may be among the wave of resignations by shadow cabinet members expected today.